Free Body Diagrams show the forces acting on a single object.
Draw this when you solve a forces problem. It helps you do the math to find the net force on the object. (You need that for Fnet = ma).
Example 1: Object pushed across a horizontal surface at constant velocity.
The box in the middle represents the object.
Each arrow is a force acting on the object, and they are all labeled with a symbol showing what type of force it is.
For an object being pushed, there is a gravity force that is canceled out by a normal force (surface pushing upward). There is also an applied force that is equal to a friction force in the opposite direction.
In this case, the magnitude (amount) of each force is also shown.
From the diagram, we can see the the forces are balanced, because the forces upward and downward are equal to each other, and in the horizontal directions things are similarly balanced.
(Balanced forces means net force is zero and acceleration is zero.)
Example 2: Object in Free Fall
This diagram shows that the object has gravity acting downward and air resistance acting upward.
We can tell that the forces are unbalanced since the arrows are different lengths.
The downward gravitational force is greater, so the net force is downward. This means the object is accelerating downward.
To calculate the net force, we would write an equation like this:
Fnet = Fg − Fair
We know that we have to subtract them because the Fg and Fair vectors are in opposite directions.
If vectors are in the same direction, we would add them instead.